The present invention relates to a curable resin composition to be used for resin processing and molding which has decomposability and does not prevent recycling of itself or materials to which it is attached, and a resin to be used for paints, inks, adhesives and the like containing the resin composition.
Currently, a large quantity of resins is used for preparing paints, inks, adhesives and resin processed moldings. These are generally used once and then thrown away, and their treatment after use such as their decomposition is not considered. Therefore, it is hindered to recycle not only the resins themselves but also the materials that are covered with these resins or to which these resins are attached.
For improving the above point, the present inventor found that the chemical structure represented by formula (1) below in a molecule could easily decompose by the contact with an acid, water and a solvent at the same time. Thereafter, a method for removing it was applied for a patent, i.e., Japanese Patent Application No. 10-219156/1998.
The present invention relates to a resin composition to which such method for such removal is applicable. At present, as a system to which the method for removal is applicable, there is paint where a resin containing a carbonyl group is cured with a dihydrazide compound, for example. However, since the chemical structure of formula (1) is formed at the curing in that method, the paint cannot be applied to the case where crosslinking is not required or where crosslinking is carried out by other means such as light, heat or the like, so that its usable range is limited.
As a result of extensive studies for improving the defect, the present inventor has found that the problem is solved by incorporating the linkage in a resin itself, and therefore has accomplished the invention.
The present invention relates to a curable composition containing a resin (A) having the structure of the structural formula represented by the formula (1) below: 
and a crosslinkable functional group or groups in one molecule. Particularly, the structure of the structural formula represented by formula (1) present in the resin (A) may be formed by the reaction of a carbonyl group with a hydrazide group or a semicarbazide group.
Further, the crosslinkable functional group (or groups) present in the resin (A) may be one type or two or more types selected from a polymerizable double bond, hydroxyl group, carboxyl group, isocyanate group, amino group, thiol group, epoxy group, triazine group, dimide group, silanol group, an alkoxysilyl group, methylol group, an alkylmethylol group, an acetoalkoxy group, carbonyl group, hydrazide group, a blocked isocyanate group, a blocked carboxyl group and a blocked hydroxyl group.
Still further, the resin (A) may be that having one or more structures of the structural formula represented by the formula (1) between the crosslinkable functional groups.
The resin may be a resin having the structure of the structural formula represented by formula (1) in the backbone of the resin and the crosslinkable functional group in the side chain.
Additionally, the resin (A) may be a resin having the same number of the structure or structures of the structural formula represented by formula (1) and the crosslinkable functional group or groups.
An organic resin which contains the structure of the structural formula represented by formula (1) and a coexisting crosslinkable functional group may be used as the resin (A). The examples of the organic resin may include such as an acryl resin, alkyd resin, vinyl resin, urethane resin, epoxy resin, polyester resin or polyether resin.
The structural formula represented by formula (1) can be decomposed in the presence of moisture, under an acidic atmosphere, or preferably in an organic solvent, and can be formed, for example, by a reaction of a compound containing carbonyl group with a compound containing hydrazide group and/or a compound containing semicarbazide group. Furthermore, the crosslinkable functional group may be introduced, for example, by using a compound containing carbonyl group or hydrazide group which also possesses the crosslinkable functional group.
Examples of the compound containing carbonyl group include such as diacetone alcohol or 4-hydroxy-2-butanone. Examples of the compound containing hydrazide group include such as iminodiacetic acid dihydrazide, adipic acid dihydrazide, carbohydrazide, glutaric acid dihydrazide, azelaic acid dihydrazide, dodecanoyldihydrazide, cyclohexanetricarboxylic acid trihydrazide, tartaric acid dihydrazide or crotonic acid dihydrazide. Diacetone acryl amide, acrolein and the like may be mentioned as the compound containing carbonyl group which further possesses a polymerizable double bond as the crosslinkable functional group.
More concretely, in the case of polymerization-type resins such as an acryl resin and vinyl resin, the resin (A) can be obtained by copolymerizing polymerizable unsaturated monomers each having the structure of the structural formula represented by formula (1) and a crosslinkable functional group, or by addition or condensation reaction of compounds each having the structure of the structural formula represented by formula (1) and a crosslinkable functional group to a polymerization-type resin. In the case of condensation-type resins such as alkyd resins and polyester resins, the resin (A) can be obtained by using a reaction product of a compound containing carbonyl group and hydroxyl group or carboxyl group with a compound containing two or more hydrazide groups and/or semicarbazide groups as at least part of polyhydric alcohol or polybasic acid. In the case of urethane resins, the resin (A) can be obtained by using a reaction product of a compound containing carbonyl group and hydroxyl group with a compound containing hydrazide group and/or semicarbazide group as a polyhydric alcohol and reacting it with a polyisocyanate. In the case of epoxy resins, the resin (A) can be obtained by adding a reaction product of a compound containing carbonyl group with a compound containing hydrazide group and/or semicarbazide group to an epoxy resin or by adding a compound having amino group or carboxyl group and hydrazide group to an epoxy group and subjecting to dehydrative condensation of a compound having carbonyl group with the hydrazide group. In the case of polyether resins, the resin (A) can be obtained in a similar manner to the above epoxy resin case using a polyether resin having epoxy groups.
The crosslinkable functional group to be introduced into the resin (A) at the same time when the structure of the structural formula represented by formula (1) is introduced is not particularly limited, and may be exemplified by a polymerizable double bond, hydroxyl group, carboxyl group, isocyanate group, amino group, thiol group, epoxy group, triazine group, dimide group, silanol group, alkoxysilyl group, methylol group, alkylmethylol group, acetoalkoxy group, carbonyl group, hydrazide group, locked isocyanate group, blocked carboxyl group and blocked hydroxyl group. One type or two or more types of these groups may be introduced in the same molecule.
The resin (A) containing a polymerizable double bond can be by itself cured with crosslinking by oxidative polymerization or irradiation with an active energy beam such as light or electron beam. The other crosslinkable functional group is preferably used in combination with a crosslinking agent (B) having a functional group reactive thereto.
The examples of the crosslinking agent (B) include polyisocyanate compounds, polyepoxy compounds, melamine resins, urea resins, benzoguanamide resins, polyoxy compounds, polycarbonyl compounds, polyhydrazine compounds, polyamine compounds, polysemicarbazide compounds, polydimide compounds, polyacetoacetoxy compounds, polysilanol compounds, polyalkoxysilyl compounds and the like. Two or more of them may be used in combination.
The curable resin composition of the present invention may contain a coloring pigment, an extender, a thickening agent, a defoaming agent, an organic solvent, water, a plasticizer, a resin other than the above, if necessary.
The method for curing the curable resin composition of the present invention may be optionally selected depending on the crosslinkable functional group of the resin (A) and the composition of the crosslinking agent (B), and the resin composition can be cured under room temperature, heating, irradiation with an active energy beam and the like.
The resin composition of the present invention thus cured with crosslinking can be decomposed by bringing it into contact with an aqueous solution of formic acid, acetic acid, p-toluenesulfonic acid, dodecylbenzenesulfonic acid, sulfuric acid, phosphoric acid or the like, and further with a solution containing an organic solvent, and thereby the cured composition can be removed.